Defaults for mandatory and difficulty

[kno10]

As I don't want any "bonus tokens", I have to turn off "mandatory question" for every single question.
It would be better to have a global user default for this (also for, e.g., disabling sound).

I also noticed a translation error: "Very easy (+ Normale x score)"

As it is not possible to file issues in arsnova.click-v2-frontend I am filing it here.
I've been experimenting with the installation at https://antworte.jetzt/ - which may or may not be in sync with this branch.